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Direct observation of early universe's higher density

  1. Mar 28, 2010 #1
    Thinking about the hypothetical concept of time expansion mentioned in https://www.physicsforums.com/showthread.php?t=389378" made me consider something for the first time:

    The primary signature of spatial expansion is, of course, redshift of the light that reaches us from the distant universe (i.e. the distant past). However, an even more immediate consequence of that expansion is the dilution of matter. What I'm wondering now is whether that dilution is directly observable, i.e. whether we observe the early (transparent) universe to be denser than today's universe. At, say, z=1, we can assume the structure of the universe to be very similar to today's (right?), but its density should be almost ten times higher. At z=4, the density should be more than a hundred times higher, but I'm not sure how far galaxy formation has progressed at this point, which might complicate matters.

    Anyway, an x times higher density should mean an x times higher concentration of galaxies and/or an x times denser IGM. Transverse to our line of sight, I imagine one would have to take optical effects into account. To resort to the inflating-balloon analogy and assuming no horizon for the moment, the highest-redshift objects we observe in any direction would be those on the far side of the balloon, i.e. we would observe the same small region stretched into a large annulus. Radially, this does not occur, but a multitude of other factors affecting the light between emission and reception have to be taken into account. So I'm guessing a direct observation is much harder to accomplish than one might naively assume... or is it?
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Mar 28, 2010 #2
    No, it was such an observation almost fifty years ago (by Penzias and Wilson in 1964) that convinced most scientists that the universe did have a beginning. The cosmic microwave background radiation (CMB) is now accepted to be very energetic photons from when the universe cooled to where atoms could form, but red-shifted down into microwave frequencies. If you look in any direction you see the same primeval fireball filling the universe with no gaps.

    There have been several satellites since, such as WMAP that have measured the tiny variations in the CMB that resulted in the large scale structure of the universe that we see today.

    Hmm. It is tough to explain the next part in words that make sense, but the physics is real. (Well, the simulations say that this is what happened. ;-) Plasma is a state of matter where a significant fraction of the atoms are ionized. Of course, in the case of hydrogen, when it is ionized you have an electron and a proton. Helium is pretty much the same way, but singly ionized helium atoms can exist. Anyway, when the universe cooled down to the point where atoms could exist, it was filled with a plasma that was 99% hydrogen and helium ions.

    That plasma absorbed photons before they could travel very far. Not very far started out less than an inch, and grew to miles as the plasma expanded and cooled. Eventually the cooling and expansion reached the point where a photon could travel a significant portion of the way around the universe. This was a very short event in terms of the total life of the universe to that point, so in effect the universe went from emitting light in all directions, to completely dark instantly. This dark age persisted until the first stars formed.

    The important point to take away from this is that the limit on how far back in time we can see is when this decoupling occurred, so seeing the CMBR is as far back as we can see.

    Oh, and something you don't see in science textbooks yet. The decoupling did not occur everywhere at the same instant. When a "cold" spot of neutral atoms and empty space formed, radiation would race across the (small) void in the plasma, be absorbed on the other side and re-emitted. If it was re-emitted in the reverse direction it raced across the void again, and bounced off the other side. (It isn't really correct to talk about the new photon as if it were the same as the old photon, but that's a detail.) In effect, how the cooling occurred was that "cold" areas (still thousands of degrees) were filled with photons that compressed the (ionized) matter into a structure like a cellulose sponge. The structure of the sponge is best described by looking at the bubbles where there is no sponge. ;-)

    Now think of one of these threadlike junctions between several bubbles. The plasma will cool rapidly because it is radiating energy into those voids. (Everything is still expanding, of course, but the bubbles are expanding even faster. ;-) Once the bubbles intersect, the emitted radiation never strikes more plasma, and the decoupling occurs. (I'd like to put a number here for how long the process took, but the best I could do would be to link to several papers. The models are good enough to show what happened, but the simulations have time steps much, much longer than the decoupling.)

    So the large scale structure of the universe today has galaxies and galaxy clusters that formed in the strands of matter and the areas where they intersect, while there are huge voids which were the slightly cooler areas in the original fireball as it cooled.
  4. Mar 29, 2010 #3
    Okay, now I feel bad, because you went to a lot of effort to tell me things I'm already familiar with. I apologize for not being sufficiently specific in my OP.

    To rephrase: What I am wondering about is our ability to observe matter density in the universe once galaxy formation has reached a point not too dissimilar from what we see today (i.e. a looong time after recombination). I'm assuming for the moment that this is extremely difficult, for the simple reason that if it weren't, it would invariably be given as the primary evidence for cosmic expansion - it's significantly more compelling than the items that are used instead (redshift, CMB, et al), which all rely on a fair amount of interpretation. And, if my suspicion is correct, why do we not have this ability, is it for the reasons I outlined above, or others I overlooked?
  5. Mar 29, 2010 #4


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    It is frightfully difficult to observe anything at z~6, so it is unclear what basis you you have in mind to overturn redshift, cmb, et al. Philosophy is insufficient.
    Last edited: Mar 29, 2010
  6. Mar 29, 2010 #5
    That post stumps me a little. Where does the z~6 come from? Why do you expect the hypothetical direct observations to overturn, rather than confirm, the less direct evidence?
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